The invention relates to a system and method for producing radio-frequency electric fields suitable for accelerating atomic and molecular ions in an ion implantation system.
The use of beams for treating and irradiating a workpiece in general and semiconductor wafers in particular is known in the prior art. Charged particles (e.g., ions of atomic and molecular species) are often accelerated to high velocities before impinging on the workpiece. In the case of ion implantation, the final energy level of the accelerated ions is controlled in order to achieve a desired depth of implantation and substrate characteristic.
In one type of accelerator, an ion beam is accelerated by a static electric field. The final ion energy is controlled by adjusting the strength of the electric field and by adjusting the distance over which the electric field accelerates the ions. In another type of static electric field accelerator, negatively charged ions are accelerated from ground potential to a high positive potential and then stripped of their electronic charge to form positive ions that are finally accelerated back to ground potential.
Recently, an ion accelerator has been developed that produces ion-accelerating electric fields that vary periodically with time at a specified rf frequency. This system is described in U.S. Pat. No. 4,667,111 to H. F. Glavish et al., which is herein incorporated by reference. According to this scheme, an accelerator is constructed from multiple acceleration stages, with each stage including an rf resonant circuit that has an accelerating electrode disposed between two grounded electrodes that are respectively spaced apart from the accelerating electrode to define accelerating gaps therebetween. A beam of ions enters the accelerator with a low initial velocity and the ions are subsequently accelerated to energies on the order of 1 MeV per charge state. Synchronous acceleration of ions results when the phase and amplitude of the rf voltage applied to each electrode is selected appropriately for the specified frequency, the geometry of the electrodes, the initial ion energy, the final desired ion energy, and the charge-to-mass ratio of the accelerated ion species. For further description of such an ion acceleration scheme see H. F. Glavish, "Radio-frequency linear accelerators for ion implanters", Nucl Instr. & Methods, vol B21 1987, pp. 218-223, and H. F. Glavish, "Radio-frequency linear accelerators for commercial ion implanters", Nucl. Instr. & Methods, vol. B24/25 1987, pp. 771-775, which are herein incorporated by reference.